Tissue Carnitine Homeostasis in Very-Long-Chain Acyl-CoA Dehydrogenase–Deficient Mice

Abstract

Deficiency of very-long-chain acyl-CoA dehydrogenase (VLCAD) is the most common long-chain fatty acid oxidation defect and presents with heterogeneous clinical manifestations. Accumulation of long-chain acylcarnitines and deficiency of free carnitine have often been proposed to play an important role in disease pathogenesis. The VLCAD-deficient mouse exhibits similar clinical and biochemical phenotypes to those observed in humans and, therefore, represents an excellent model to study VLCAD deficiency. We measured carnitine and acylcarnitine profiles in liver, skeletal muscle (SkM), bile, and blood from VLCAD knock-out mice and controls under nonstressed and various stress conditions. Carnitine and acylcarnitines were extracted from body fluids with methanol and from tissues with acetonitrile, respectively, and were analyzed as their butyl esters using electrospray ionization tandem mass spectrometry. Fasting combined with a cold challenge for 8 h significantly induced liver long-chain acylcarnitine and free carnitine production. Acylcarnitines in SkM predominantly accumulated during exercise with a concomitant decrease of free carnitine. Changes in blood free carnitine did not correlate with carnitine homeostasis in liver and SkM. Our results demonstrate different tissue-specific long-chain acylcarnitine profiles in response to various stressors, which may be of importance with respect to the heterogeneous clinical manifestations of VLCAD deficiency in humans. Furthermore, we conclude that carnitine biosynthesis in the liver seems sufficiently active to maintain liver carnitine levels during increased demand. Our data suggest that carnitine supplementation in long-chain ␤-oxidation defects may not be required, and blood carnitine concentrations do not reflect tissue carnitine homeostasis. Deficiency of very-long-chain acyl-CoA dehydrogenase (VLCAD) is the most common mitochondrial ␤-oxidation defect of long-chain fatty acids, with an incidence of~1:50,000 to 1:100,000 births (1). In humans, three distinct clinical phenotypes of different severities occur: a life-threatening, early-onset presentation with cardiomyopathy and hepatopathy; a hepatic phenotype with recurrent hypoketotic hypoglycemia and onset in infancy; and a milder, myopathic form with episodic muscle weakness, myalgia, and rhabdomyolysis presenting in adolescence or adulthood (2). Treatment interventions such as avoidance of fasting, reduced long-chain fat intake, and supplementation with medium-chain triglycerides may avoid metabolic decompensation, and long-term rehabilitation can be achieved (3,4).Characteristically, long-chain acyl-CoAs accumulate in mitochondria. To leave the mitochondria, they are converted into acylcarnitine esters, which can be assayed in blood (5). As a result of an increased production of acylcarnitines, blood free carnitine concentrations may decrease (5), a condition termed secondary carnitine deficiency. Nevertheless, supplementation of exogenous carnitine to restore intracellular carnitine pools has been controversia...

Prolonged QTc Interval in Association With Medium-Chain Acyl-Coenzyme A Dehydrogenase Deficiency

“…Unfortunately, it is difficult to ascertain the full clinical magnitude of this finding because recent mouse models of VLCAD deficiency have not revealed a correlation between blood and tissue free carnitine concentrations and blood concentrations are inadequate markers of carnitine biosynthesis. 14,15 There are several additional clinical observations and associations reported in MCAD-deficient patients that merit discussion. Laboratory findings during an acute metabolic crisis commonly include hypoketotic hypoglycemia, elevated uric acid, increased anion gap, elevated transaminases, and mild hyperammonemia.…”

Carnitine Supplementation Induces Acylcarnitine Production in Tissues of Very Long-Chain Acyl-CoA Dehydrogenase-Deficient Mice, Without Replenishing Low Free Carnitine

“…VLCAD Ϫ/Ϫ mice were generated as described (6,12). Genotypes were determined by duplicate PCR analyses (13).…”

“…Physical exercise or fasting, however, may induce similar clinical phenotypes to humans (11). VLCAD Ϫ/Ϫ mice present with stress-induced hypoglycemia, skeletal myopathy, and cold intolerance associated with elevated C14 -C18 acylcarnitines (6). Overall, the VLCAD Ϫ/Ϫ mouse is an excellent model to study human VLCADD.…”

“…As a result of an increased production of acylcarnitines, blood free carnitine concentrations may decrease (5). Especially physical exercise results in decreased free carnitine concentrations in skeletal muscles (6). However, earlier studies have shown that blood and tissue concentrations of free carnitine do not always correlate (6).…”

“…Especially physical exercise results in decreased free carnitine concentrations in skeletal muscles (6). However, earlier studies have shown that blood and tissue concentrations of free carnitine do not always correlate (6). It has been widely discussed whether supplementation of exogenous carnitine is advisable to recover intracellular carnitine concentrations (7,8).…”